1. Field of the Invention
This invention relates generally to machines for separating liquid-solid suspensions continuously supplied to a moving filter. Although this invention can be employed to dewater a wide variety of suspensions, it has been found to be especially useful in separating water from mixtures containing a high percentage of ultrafine particles.
2. Description of the Prior Art
As byproducts of sand and gravel operations, including those preparing aggregates for concrete work and for road construction, industry generates tremendous quantities of ultrafine particles, commonly referred to as fines, in the form of suspended silts and clays. Unfortunately, there is little market for such materials per se; and they can be mixed only in very limited quantities with the rest of a saleable product. Indeed, of the total amount of silts and clays generated annually, only about 5 to 10 percent is being sold. The bulk of the annual production of suspended silts and clays is simply discarded.
Already, the volume of fines generated each year is staggering. In a single year, a typical plant can extract 500,000 tons of limestone from an 1 acre deposit and simultaneously produce, on a dry weight basis, 120,000 tons of fines. Moreover, reserves that were formerly considered uneconomical to mine because of their high silt or clay content are being tapped, increasing the ratio of fines-to-product generated annually.
The problem of what to do with the fines is expected to become more intractable, as long as prior art technology is employed, not only in terms of total volume but also in terms of increased unit cost per ton of fines disposed. This unit disposal cost has risen as plants have become more efficient at extracting a higher percentage of coarse materials from the residuals containing the fines.
The trend, at least in larger plants, is to employ cyclone separators to remove, from suspended silts and clays, a high percentage of those particles which are too large to pass through a 200 mesh screen. Experience has shown that downstream of such a cyclone separator, a suspension tends to be more difficult to dewater than it otherwise would have been.
Difficulties in dewatering a suspension of fines, even if it has not been subjected to a cyclone separator, are encountered once the suspension to allowed to settle, whether on a filter medium or in a drying bed. Upon settling, the suspended particles closest to the filter media or, alternately, to the bottom of the drying bed, tend to compact together, forming a substantially impermeable layer of suspended particles. While this lowest layer has a relatively low liquid content, successive layers of suspended particles disposed upwardly thereof do not.
The problem of dewatering a suspension trapped above a compacted layer of suspended particles increases dramatically whenever this layer is formed from a relatively homogeneous suspension which has a high percentage of colloidal-size particles (the so-called "ultrafines"); in such a situation, bonding between the particles is inhibited, reducing their settleability.
Percolation of water through a drying bed containing a slurry of fines in which little coarse material is present can be so slow that a virtual permanent quicksand is created. Not only does this quicksand give rise to a serious hazard to human beings and wildlife but also extensive environmental degradation results. Even an average-size plant, producing silt in suspension at the rate of 15 tons (dry weight) per hour, for example, consumes 25 acres per year in "drying beds".
On the other hand, attempts to force liquid to move, under pressure, through a compacted layer of suspended particles juxtaposed against a filter medium, except in small batch-type operations utilizing a pressurized chamber, have been without appreciable success. In particular, when a higher rate of feed must be handled than can be accommodated with a filter press, operators typically rely on a belt press, that is, a dewatering system utilizing an endless perforate conveyor belt. Unfortunately, when pressure is applied to a mixture of suspended particles on such a belt, portions of the mixture usually flow to the sides of the belt.
The prior art has sought to enhance drainage of filtrate through perforate belts with the use of plows. Extending downwardly into the suspended mixture, most of these plows are stationary and have working faces that are disposed perpendicularly to the upper surface of the belt; any compacted layer of solids agglomerated thereon which is brought into contact with these faces is dislodged horizontally, exposing the filter surface. Exceptions are the rotary gang plows taught by Carmel in U.S. Pat. No. 3,891,549.
Using his plows in a method for dewatering inhomogeneous suspensions having a high initial liquid content and an abundance of coarse particles, Carmel specifies that the plows must be rotated to prevent the formation of clods. Alternating with Carmel's plows are rollers which also rotate. The latter rotate concurrently with the direction of movement of the perforate conveyor belt and have a circumferential speed exceeding its speed.
With their working faces disposed at an angle substantially less than 90 degrees to the belt surface, Carmel's plows not only penetrate the lower compacted layer of solids, ditching it and exposing the filter surface but also lift the compacted layer, turning the suspended mixture at least partially upside down. In the process, a layer of coarse particles of greater porosity than the compacted layer is brought into close proximity with the filter medium, facilitating drainage of liquid formerly trapped above the compacted layer. Downstream of the plows, the rollers squeeze more liquid out of this layer of coarse particles and mix them together with the finer particles.
Unfortunately, the dewatering of slurries with a high percentage of ultrafines is complicated by the need to use chemicals such as polymers to promote settling through the formation of aggregates of particles. Without aggregate formation, 80-90 percent of the suspended solids present in such a slurry would pass through a 325 mesh screen, i.e., they are as fine as grains of flour.
Both the rotary plows and rollers of Carmel's combination and the perpendicularly-disposed working faces of the stationary plows tend to rupture bonds between polymers and suspended solids. Missing from the prior art are any methods and devices for promoting aggregate formation by particle bonding within suspensions even as they are being dewatered on a moving conveyor belt.